<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html,
        body {
            font-size: 18px;
        }

        @media only screen and (max-width: 1080px) {

            html,
            body {
                font-size: 16.875px;
            }
        }

        @media only screen and (max-width: 960px),
        only screen and (max-device-width:960px) {

            html,
            body {
                font-size: 15px;
            }
        }

        @media only screen and (max-width: 800px),
        only screen and (max-device-width:800px) {

            html,
            body {
                font-size: 12.5px;
            }
        }

        @media only screen and (max-width: 720px),
        only screen and (max-device-width:720px) {

            html,
            body {
                font-size: 11.25px;
            }
        }

        @media only screen and (max-width: 640px),
        only screen and (max-device-width:640px) {

            html,
            body {
                font-size: 10px;
            }
        }

        @media only screen and (max-width: 600px),
        only screen and (max-device-width:600px) {

            html,
            body {
                font-size: 9.375px;
            }
        }

        @media only screen and (max-width: 540px),
        only screen and (max-device-width:540px) {

            html,
            body {
                font-size: 8.4375px;
            }
        }

        @media only screen and (max-width: 480px),
        only screen and (max-device-width:480px) {

            html,
            body {
                font-size: 7.5px;
            }
        }

        @media only screen and (max-width: 414px),
        only screen and (max-device-width:414px) {

            html,
            body {
                font-size: 6.46875px;
            }
        }

        @media only screen and (max-width: 400px),
        only screen and (max-device-width:400px) {

            html,
            body {
                font-size: 6.25px;
            }
        }

        @media only screen and (max-width: 375px),
        only screen and (max-device-width:375px) {

            html,
            body {
                font-size: 5.859375px;
            }
        }

        @media only screen and (max-width: 360px),
        only screen and (max-device-width:360px) {

            html,
            body {
                font-size: 5.625px;
            }
        }

        @media only screen and (max-width: 320px),
        only screen and (max-device-width:320px) {

            html,
            body {
                font-size: 5px;
            }
        }

        @media only screen and (max-width: 240px),
        only screen and (max-device-width:240px) {

            html,
            body {
                font-size: 3.75px;
            }
        }

        .one {
            height: 6rem;
            background-color: red;
        }
    </style>
</head>

<body>
    <div class="one">

    </div>
</body>

</html>